Which legacy devices are still at risk when PSTN/POTS lines are retired or moved to VoIP?

The UK government's guidance on the move to digital landlines gives a list. For homes it names alarm systems, telecare devices and fax machines. For businesses it adds "lift alarms, burglar alarms or card payment systems." Telecare is the British term for the pendant and base station sold in the US as a medical alert system.

Commercial buildings add a few more. Atlantech Online, a carrier that replaces copper lines in buildings, covers fire alarm panels, elevator emergency phones, intrusion panels, gate callboxes, fax machines, point-of-sale terminals and ATMs on its POTS replacement page. It also lists the modems that monitor HVAC systems, generators and fuel tanks. By its count, a 50,000-square-foot office building usually has six to twelve analog lines.

Some of these lines run with nobody attending them, which makes them easy to overlook. The UK guidance says "Communications providers will not know which devices are connected to their network," which leaves the review to the customer. Atlantech starts a replacement by walking the building to find every analog line, the device on it and the carrier that bills for it. Its page says this is the step where surprises turn up, such as a modem that no one can find or a fax line still on the bill years after the machine was unplugged.

The first is that the line disappears. AT&T said in April 2026 that it plans to retire most of its copper network by the end of 2029, with service ending in selected areas from summer 2026 after customers are notified. In the UK, Openreach still plans to close its legacy phone network on January 31, 2027. Each carrier sets its own schedule, so no single date covers every line. We cover the wider picture in whether the PSTN is still used today.

A line can also go without a schedule. Atlantech describes a Washington, DC office building that lost all of its POTS lines when the local carrier's infrastructure was cut, and the carrier declined to restore them. Those lines served the elevator phones, fire alarm panels and security systems. By Atlantech's account the building went into fire watch, with extra staff on site around the clock, until replacement lines were working.

The second is signaling. A phone line was built for speech, and people cope with a clipped syllable. A fax machine or an alarm dialer sends data as sound, and the equipment at the far end has to decode it. When the line becomes a VoIP service, that sound is turned into packets and back again, and the conversion has to suit the device. Fax shows why this needs checking. Cisco's data sheet for its VG400 analog gateway documents two different ways of carrying a fax, T.38 relay and pass-through. The gateway is set up for one of them, and the choice has to match the service on the other side. So a new line that gives a clear dial tone has passed only the first test. The device still has to complete a real transmission to the far end.

The third is power. The UK guidance explains that an analog line carries a low-voltage supply from the telephone exchange, enough to run some basic corded handsets. A digital landline delivered over broadband through a router or adapter cannot carry that supply. Handsets and routers then run on the building's power and stop in a power cut unless a battery or generator backs them up. A panel or base station with its own supply and battery can keep the device itself powered. The line is a separate matter, because it now depends on equipment in your building. A router or adapter with no battery takes the line down with it, whatever powers the device behind it.

Ordinary phones are the simpler case. The UK guidance expects the change to be straightforward for most consumers and businesses. A person on a call notices a problem and says so, which a dialer in a closet cannot do. The power change still applies to them. A basic corded phone that ran on the line's own power stops working in a power cut once the line arrives through a router, unless that equipment has a battery.

Equipment that reports another way has less at stake. A fire panel with a cellular communicator, or a card terminal on the office network, does not place its calls over the analog line that is being retired. That holds only while its own connection stays up, so check two things. An IP device may run on broadband that arrives over the same copper. Some panels also keep an analog line as a fallback path.

An analog device is not automatically stranded either. Bay Alarm Medical's guide to compatible phone services tells customers with VoIP or cable phone service to look for systems labeled as compatible with "digital phone service." It also describes the case that does not work. Its base station plugs into a phone jack, so a home phone that reaches the modem only over Wi-Fi gives the unit nothing to connect to. For that home, Bay Alarm points to its base station with a built-in cellular connection. That is one vendor describing its own equipment. For this unit, the answer came from the company that supplies it.

Each analog device leaves you two choices. You can replace it with a version that reports over cellular or IP. The UK guidance advises checking that a new alarm or device works with VoIP services or can connect to a mobile service such as 4G. Or you can keep the device and give it a line that behaves like the old one.

Atlantech's POTS replacement is an example of the second choice. Each analog line moves to an FXS port on an appliance in the telecom closet, so the device is still given an analog line with dial tone. Our answer on PSTN gateways and FXS and FXO ports explains that port. The appliance sends calls over broadband, with 4G LTE as a second path, and Atlantech lists a battery that runs up to 24 hours. One unit handles two to eight lines. The page gives a typical cost of $15 to $45 per line per month plus hardware. Actual pricing is by quote and depends on line count, connectivity and installation.

Suppose a four-story medical office has four copper lines: a fire alarm panel, an elevator phone, a fax machine and a card terminal at the front desk. The payment processor offers a terminal that uses the office network, so that line is cancelled and never replaced. Assume the alarm company and the elevator contractor confirm that the fire panel and the elevator phone are compatible with an analog replacement line. Those two move to ports on an appliance with a battery and cellular backup, and each passes a test arranged with its monitoring center. The fax machine moves to the same appliance, set to the fax mode the service supports, and it sends and receives a test page.

A dial tone does not show that a device's signals arrive, so each device needs a test of its real job across the new line. Atlantech's cutover process says "fire and elevator lines are tested end-to-end with the monitoring center after each cut." Hold any replacement to that standard, whoever supplies it. A fax machine gets the same treatment, with a test page sent and a test page received.

Arrange the test with the alarm company or elevator contractor beforehand, so the monitoring station expects the signal and logs it as a test. A station that receives an unannounced alarm signal can treat it as a real emergency.

A passed test shows that the signal path worked on that occasion. The battery and the backup connection need their own checks.

Fire alarm and elevator lines can also carry approval requirements that depend on where the building is. Ask the alarm company or elevator contractor to identify which approvals apply to yours. Where any do, get the new path approved before you cancel the copper line, and keep that line in service until every device on it has passed its test.